Group XIII selenide materials are semiconductors with a wide variety of potential applications that include photovoltaic and data storage devices, nonlinear optics and photosensors such as p-n junction photodiodes, owing to their wide band gap ranges. The selenides exist in two oxidation states, namely, +2 MSe (M=Ga, In) and +3 M2Se3 (M=Al, Ga, In), offering a range of direct band gaps (1.4-2.5 eV for indium selenide: α-In2Se3 1.42 eV, β-In2Se3 1.55 eV, γ-In2Se3 2.00 eV; 1.8-2.6 eV for gallium selenide: α-Ga2Se3 2.2 eV, β-Ga2Se3 2.3 eV; 3.1 eV for Al2Se3).
Group XIII selenide materials can be used to fabricate photovoltaic devices, such as copper indium gallium diselenide/sulfide (“CIGS”) solar cells. For commercial viability, photovoltaic cells must generate electricity at a cost that is competitive with fossil fuels. To meet these cost constraints, the photovoltaic cells must utilize low cost materials and device fabrication and must convert sunlight to electricity with high efficiency. Materials synthesis and device fabrication must also be commercially scalable.
For photovoltaic cells fabricated from thin films, the material cost is intrinsically low, since only a small amount of material is required to produce the active layer of up to a few microns. Thus, much development work has focused on producing high efficiency thin film solar cells. Chalcopyrite CIGS devices have demonstrated significant potential. The band gaps of CuInS2 (1.5 eV) and CuInSe2 (1.1 eV) match well to the solar spectrum, predisposing them to high conversion efficiency. Currently, efficiencies up to 20.3% have been achieved for Cu(In, Ga)Se2.
Binary chalcogenide nanopowders, including copper selenide, and/or indium selenide, and/or gallium selenide, have been proposed as starting materials for CIGS solar cells. For example, U.S. Patent Publication No. US 2007/0092648 A1, entitled “Chalcogenide Solar Cells,” describes Group IB-chalcogenide nanoparticles and/or Group IIIA-chalcogenide nanoparticles as a precursor material for forming a film of Group IB-IIIA-chalcogenide compounds.
Fabrication of absorber layers for photovoltaic devices typically involves expensive vapor phase or evaporation techniques. Alternatively, nanoparticles can be melted or fused together to form a thin film, such that the nanoparticles coalesce to form large grains. For example, films of metal oxide nanoparticles can be reduced using H2 and then selenized, usually with H2Se. As described in co-owned PCT patent application published as WO 2009/068878 A2, incorporating a selenium source into the nanoparticles can obviate the costly selenization step, thereby obviating the need to use toxic H2Se.
For nanoparticles to be viable as a starting material for photovoltaic thin films, they preferably possess a number of properties. Primarily, the nanoparticles must be small, preferably of a scale such that the physical, electronic and optical properties of the nanoparticles may differ from those properties of the bulk material. By restricting the electronic wave function to such small dimensions, a particle becomes “quantum confined,” whereby its properties are intermediate between those of the bulk material and an individual atom. Such nanoparticles are termed “quantum dots.” Smaller particles are able to closely pack, enabling them to coalesce more easily upon melting. Secondly, the nanoparticles are preferably of a narrow size distribution so that the particles all melt at approximately the same temperature. This property ensures that the resulting thin film is of high, even quality. Thirdly, the nanoparticle may be capped with a volatile organic capping ligand to help solubilize the nanoparticle in solution. But the capping ligand must be easily removed upon heating to avoid the detrimental incorporation of carbon into the film.
There are a number of techniques have been used to prepare indium selenide nanoparticles. Examples of the synthesis of gallium selenide nanoparticles are less well documented, and aluminum selenide nanoparticle synthesis does not appear to exist in the art. Nanoparticle syntheses include colloidal methods, single source precursor methods, sonochemical methods, metathesis in liquid ammonia, chemical vapor deposition, and thermal evaporation.
Typically, colloidal methods involve high temperature (>250° C.) syntheses, forming nanoparticles capped with trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), phosphonic acids or amines. The resulting nanoparticles are small (<20 nm) and monodisperse, lowering their melting point with respect to that of the bulk material, thus enabling lower temperature device processing.
TOP- or TOPO-capped GaSe and InSe nanoparticles can be prepared colloidally using trimethylgallium (GaMe3) and trimethylindium (InMe3), respectively, as the metal sources. Tu, et al., describe a colloidal method for preparing GaSe and InSe nanoparticles (H. Tu et al., Nano Lett., 2006, 6, 116). Se is dissolved in TOP (for TOP-capped nanoparticles) or a mixture of TOP and TOPO (for TOPO-capped nanoparticles). After heating to 280° C., a 1 M solution of trimethylgallium or trimethylindium in TOP is injected, then the nanoparticles are grown at 268° C. for a given time. The GaSe nanoparticles are grown to 2.7 nm in 18 minutes, while the growth time for 3 nm InSe nanoparticles is 50-90 minutes, depending on the reaction conditions.
Yang, et al. describes a similar method for producing InSe nanoparticles capped with TOP and/or TOPO or with hexyl phosphonic acid (S. Yang et al., J. Phys. Chem. B, 2005, 109, 12701). The particle sizes range from 2.9 nm to >80 nm depending on the reaction temperature and time. Luminescent GaSe nanoparticles with a photoluminescence quantum yield of about 10% can be made using a similar method, as described by Chikan, et al. (V. Chikan et al., Nano Lett., 2002, 2, 141). TOP and TOPO are heated to 150° C. overnight. TOP/Se is added and the mixture is heated to 278° C. A solution of GaMe3 in TOP is injected, causing the temperature to drop to 254° C. The solution is heated to 266-268° C. and held at that temperature for approximately two hours. Upon cooling, the nanoparticles can be isolated in air. The average particle size of the resulting GaSe nanoparticles is between 2-6 nm. The size distribution can be narrowed by size-selective precipitation or chromatographic purification to yield nanoparticles that emit with a photoluminescence maximum between 320-450 nm.
Single source precursor methods use a single source molecule containing the elements of the desired nanoparticles. The source molecule is thermally decomposed to form the nanoparticles. Single source precursor methods can be used to make indium selenide nanoparticles (N. Revaprasadu et al., J. Mater. Chem., 1999, 9, 2885). TOPO- and 4-ethylpyridine-capped InSe nanoparticles having diameters of 5.8-7.0 nm have been synthesized using the single source precursor tris(diethyldiselenocarbamato)indium(III) ([In(Se2CNEt2)3]). To prepare TOPO-capped nanoparticles, the [In(Se2CNEt2)3] is dispersed in TOP, then injected into TOPO at 250° C. After an initial drop in temperature, the solution is held at 240° C. for 30 minutes. To synthesize 4-ethylpyridine-capped InSe, [In(Se2CNEt2)3] is stirred for 1 hour at 100° C. in 4-ethylpyridine. The temperature is raised to 167° C. and the mixture is refluxed for 24 hours.
Sonochemical synthesis of nanoparticles involves the ultrasonication of precursor materials in a solvent. Bulk GaSe single crystals can be converted to nanoparticles by ultrasonication (K. Allakhverdiev et al., Phys. Stat. Sol. (a), 1997, 163, 121). After sonicating the crystals in methanol for 60 minutes under ambient conditions, nanoparticles of <30 nm are obtained.
Liquid mediated metathesis reactions are those in which two chemical species exchange in the presence of a liquid medium, such as ammonia. Spherical Ga2Se3 and In2Se3 nanoparticles with diameters <50 nm can be generated by metathesis of Group XIII chlorides and Na2Se in liquid ammonia (G. A. Shaw et al., Inorg. Chem., 2001, 40, 6940). In a typical reaction, 2GaCl3 or 2InCl3 react stoichiometrically with 3Na2Se in liquid ammonia to form an amorphous phase. The solid can be annealed at 250-300° C. for 2-36 hours to form a crystalline phase. The nanoparticles produced by this method typically form large aggregates, up to several microns in diameter.
Chemical vapor deposition (CVD) is used to make thin films of semiconductor materials. InSe and GaSe nanoparticles can be formed by CVD. InSe nanoparticles have been grown on KBr substrates at 290° C. in a static environment from the thermolysis of [(EtMe2C)InSe]4. The resulting nanoparticles have a spherical morphology and an average diameter of 88 nm. Similarly, using [(tBu)GaSe]4, GaSe nanoparticles with an average diameter of 42 nm can be grown at 335° C. under argon flow. The resulting nanoparticles have a pseudo-spherical structure described as a ‘string of pearls’ structure (S. Stoll et al., Chem. Vap. Deposition, 1996, 2, 182). Under these conditions, nanoparticle formation is typically complete within 20 minutes.
Thermal evaporation involves depositing a thin film of a material by evaporating a bulk source of the material under the flow of a carrier gas, which is then deposited on a substrate upon cooling. In2Se3 nanowires have been synthesized by thermal evaporation techniques, whereby bulk In2Se3 is loaded into a tube furnace under the flow of a carrier gas. The furnace contains a silicon substrate covered with a gold catalyst upon which nanoparticles are grown by ramping the temperature. For example, Li et al. describe the growth of indium selenide nanowires using an argon carrier gas and ramping the source to 940° C. and the substrate to 690° C., maintaining the temperature for 90 minutes (Y. Li et al., J. Mater. Chem., 2011, 21, 6944). In2Se3 nanowires formed in 2% H2 in N2 gas at 60 mTorr at 700° C. have also been reported (H. Peng et al., J. Am. Chem. Soc., 2007, 129, 34).
Methods of forming films of Group XIII selenide nanoparticles must be economically competitive if devices using such films are to be viable. Such methods include, but are not limited to, printing or spraying processes. The examples of Group XIII selenide synthesis described in the prior art are unfavorable for preparing materials for processing into thin films on a commercial scale because those prior art methods do not provide the essential features described above. Namely, the prior art does not provide a scalable reaction to generate Group XIII selenide nanoparticles with a low melting point, narrow size distribution and a volatile capping ligand. For instance, hot injection techniques produce materials in very low yields and are not easily scaled commercially. Other techniques like sonochemical synthesis and liquid metathesis do not allow a tight control over the physical properties of the nanomaterials, including particle size and monodispersity.
Solar cells that can be printed on flexible substrates represent an attractive cost-efficient alternative to the conventional vacuum-deposited solar cells because the materials can be deposited by using non-vacuum, solution-processable printing technologies. Thus there is a need for a simple, low-temperature technique for fabricating high-quality and uniform M-Se material (M=Ga, In) that can be easily dispersed into aqueous and organic media enabling fast and economic device manufacturing using solution-processable deposition techniques to meet the growing demand for low-cost solar cells on flexible substrates. In addition to their use in photovoltaic devices, Group XIII selenide nanoparticles may be attractive as an alternative to organic dyes used in optical information data storage applications.